Voters queue to cast their ballots during the general election at a polling center in Singapore September 11, 2015Reuters

Singapore President Tony Tan Keng Yam issued a writ of election on Wednesday confirming the Bukit Batok by-election will be held on May 7.

The nominations will take place on 27 April at Keming Primary School. The election day will not be a public holiday, but the government asked employers to allow reasonable time for people to cast their votes.

According to the elections department, there are 25,616 voters in the Bukit Batok single member constituency.

Prime Minister Lee Hsien Loong exhorted Bukit Batok residents to elect a candidate who will ably represent them in parliament.

"Partnership between Government and citizens is key to our success. I hope Bukit Batok residents will elect the candidate who will be concerned for their welfare, represent them ably in Parliament, and make Bukit Batok the best home for them, Lee said in a Facebook post.

The Bukit Batok by-election was necessitated after People's Action Party (PAP) MP David Ong resigned in March after admitting to an extra marital relationship.

Ong had won the 2015 general election with 73 percent of votes but observers believe the by-election will be a closer fight.

PAP chose lawyer Murali Pillai as its candidate for Bukit Batok Single member constituency. Murali is the second minority candidate fielded by the PAP in a single seat.

His chief rival will be Singapore Democratic Party (SDP) secretary-general Chee Soon Juan. Though Ong defeated SDp candidate by a wide margin last year, the candidacy of Chee brought to the table a stronger fight in a constituency where the ratio of Chinese population is higher than the national average.

Chee exuded confidence in a recent Facebook post saying he and his party will surmount odds to win the Bukit Batok seat. "We've got a mountain to climb and we will climb it," he said.

In the latest development, independent candidate Samir Salim Neji, who got a paltry 0.6 percent votes in the last election, pulled out of the fray.

David Ong, who resigned in March citing a "personal indiscretion", later sent residents a letter of apology. Ong says he is deeply sorry that he let down the residents by having to resign.

Ong resigned as MP and quit the ruling party after Chinese daily Lianhe Zaobao revealed his affair with the 41-year-old woman, a grassroots PAP member and a logistics firm employee. The daily said the husband of the woman allegedly lodged a complaint over the affair.